Pour the remaining tablespoon of oil into the wok, and stir in the carrots, broccoli, and water chestnuts. Cook and stir until the chestnuts are hot, about 5 minutes more. Stir up the sauce to redistribute the cornstarch, then pour into the wok, and bring to a boil. Add the reserved chicken, and stir until the sauce thickens, and the chicken is hot. Sprinkle with cashews to serve.